Back to learning this. (Been too busy at work and I forgot EVERYTHING.)
Hopefully, you’ll see a screen shot in this post, and as you’ll also see, the vertices on the open end of this half of the pill are selected. Tutorial tells me to go to Mesh…Faces…Fill. But it ain’t doing a thing.
What am I doing wrong?
Solved, but don’t ask me how the heck I did it.
Well, if you look closely, you’ll see that on the left and right side of the selected part, you have some “double” vertices (well, some groups of vertex close to each other, creating a kind of X shape). I think that was was puzzled Blender and made it unable to automaticaly fill the circle - because the surface to fill already had some faces (in the X) and wasn’t “flat” enough.
Something like that 
Can’t explain better, sorry.
Thanks for replying, Rore. These tutorials are a b****.